Devices for compression of the mammalian chest and/or abdomen or application of externally applied pressures to a body surface have been used extensively on patients for many medical purposes. The most common example is in cardiopulmonary resuscitation (CPR). For manually applied CPR in human adults a mid-sternal chest compression of 1½ to 2 inches at a rate of 100 per minute is recommended by the American Heart Association. For a normal adult chest elasticity this requires a force of approximately 100 lbs. A common complication attributed to this high force is rib or sternum fracture. Prior studies on accidental injury have reported that chest deflections as little as 2.3 inches have resulted in rib fracture (J. Cavanaugh, In: Accidental Injury Biomechanics and Prevention, 2nd edition, Eds. A. Nahum and J. Melvin, 2001, Springer-Verlag, pg 377). It is also well known that the elderly are more prone to fractures. Manual CPR involves applying the entire force through the palm of one hand. This is a similar force application to automated CPR machines such as the Thumper (Barkalow, U.S. Pat. No. 3,364,924) or Lucas (Hampf, U.S. Pat. No. D461,008, Steen, U.S. Pat. No. 7,226,427) systems. Another automated CPR system called the AutoPulse (Sherman, U.S. Pat. No. 6,616,620) applies the force through a pad pulled over the anterior surface of the chest via motorized belts. These automated systems are all in common use. Halperin et al. (Halperin, U.S. Pat. No. 4,928,674) described an automated inflatable cuff surrounding the anterior and lateral surfaces of the chest which then resulted in what they considered was a uniform circumferential compression. This resulted in less rib compression required for a given volume or intrathoracic pressure change. These investigators were then able to apply considerably higher total force for the same chest deflection. One reason for this improvement was the constraint set by the posterior wall of the thorax since it is fixed by the spine and is not involved in chest volume change. Thus, any applied force limited to the anterior chest surface as in Manual CPR, Thumper, Lucas or AutoPulse leads to bulging of the unconstrained lateral chest surfaces (and loss of compression) which is prevented by uniform circumferential compression. Uniform compression also avoids stress concentrations such as is obvious near the sternum with manual CPR or Thumper or Lucas and at the borders of the anterior and lateral surfaces when a pad is used over the anterior surface. Such stress concentrations increase the likelihood of fractures. Despite stress advantages, the cuff system requires a cumbersome pneumatic system which is impractical for portable emergency use. This limitation is due to the large bladder size (volume) required to surround the chest and the need to rapidly inflate and deflate this bladder up to 100 times a minute. Also, a cuff system applies a constant pressure during compression unlike a volume reduction due to natural muscle which reduces force as volume decreases because of the length-tension property of muscle. Natural muscle applies maximum force initially at resting length and reduces force as it shortens even with a constantly maintained stimulation. At close to 50% shortening of the initial resting length net muscle force decreases to zero. This means that a cuff imposed compression will result in a higher mechanical stress compared to what is possible with natural muscle.
Alternating compression and decompression of the thorax and/or abdomen is an old idea credited to R. Eisenmenger (Wien Klin Wochenschr 42: 1502-3, 1929) which has recent device manifestations. Both active and passive decompression has been tried. Merely binding or applying a constant pressure over the abdomen during chest CPR has also been found to be advantageous (Lottes et al. Resusitation: 75: 515-24, 2007). Cyclical compression of the abdomen alone has also been tried in animals and found to lead to improved indices of coronary blood flow (Geddes et al. Am. J. Emerg. Med. 25: 786-790, 2007). A limitation of any form of abdominal compression is the possible consequences of a full stomach during compression for CPR purposes. A mouthward movement of stomach contents could compromise application of assisted ventilation which is usually simultaneously required.
Another motivation for manipulating externally applied pressures is to assist cough or relieve choking. The most well known example is the Heimlich maneuver which uses manually applied abdominal pressure to dislodge food from the airway. Application of a relatively high vacuum at the mouth (machine exsufflation) for short durations following a large inspiration (machine insufflation) is a cough assist technique developed in the 1950s for the polio epidemic and has recently been brought back for patient use (Be'eri, U.S. Pat. No. 7,096,866). Airflow levels lower than normal cough results with this method. This technique requires application of large positive and negative pressures at the mouth (up to + or −45 cm H2O) which is not well tolerated by all patients. Bach, who has been a primary force in re-emergence of this technique, (Chest 126: 1388-1390, 2004) has recommended that: “ . . . we have always found it extremely important to institute abdominal thrusts during the exsufflation cycling of the machine to maximize cough flows.” The abdominal thrusts refer to manually applied compressions which could also be applied by an assistive device. Simultaneous compression of the chest and abdomen along with voluntary closure followed by sudden opening of the glottis is a possible assist method to use on a repeated basis which is much closer to normal cough than the machine in-exsufflator. Such compression can also be used in combination with manipulation of mouth, mask, or tracheal pressure using the in-exsufflator machine to further enhance cough airflow. The in-exsufflator machine is apparently only well tolerated by 90% of patients (Miske et al. Chest 125: 1406-1412, 2004) so alternative assist devices are needed as well. Electrical stimulation of abdominal muscles has been tried for cough assist (Linder, U.S. Pat. No. 5,190,036), but the level of airflow does not match a normal cough. In addition, direct electrical stimulation of muscle has the added complication of pain fiber stimulation which limits the magnitude of tolerable assist. Cough assist is important to patients with spinal cord injuries who lack chest muscle control or elderly people too weak to cough effectively.
The Valsalva maneuver is a common voluntary practice where contraction of the abdominal muscles while closing the glottis is used to increase intra-abdominal pressure and aid peristalsis in propelling stool during defecation or bladder emptying. Spinal cord injuries are the most common cause of problems associated with bowel movement or bladder emptying. Constipation, digestive tract disease, and age are other examples where abdominal muscle function may be inadequate. Any assist to the abdominal muscles such as discussed above for cough can also be used for this purpose. Similar to cough this is most effectively done with participation by the subject in synchronizing assist with glottic aperture closure. The cardiovascular response to the Valsalva maneuver is different for normals and patients with heart failure (Felker et al. Am J. Med. 119: 117-122, 2006). This difference has been applied as a basis for using the Valsalva maneuver as a diagnostic test of cardiovascular function. The Valsalva maneuver using a facemask and valve applied during expiration has also been proposed as an aid for pressure equilibration at altitude (Ansite, U.S. Pat. No. 5,467,766). No assistive chest or abdominal compression was used for this device.
All prior methods applied for chest compression are very different than the normal physiological manner of reducing chest volume during expiration which involve shortening of muscle fibers between adjacent ribs. The external intercostal muscles run obliquely (downward and forward) from each rib to the rib below and attach to the outer surface of the ribs. The internal intercostal muscles attach to the inner surface of the ribs and run at right angles to the external intercostals. Co-ordinated contraction of internal and external intercostal muscles will lead to chest compression or expansion by drawing certain ribs together and being constrained by the structural arrangement of the ribs. Since all rib pairs have these muscles volume changes are accomplished very evenly and without stress concentration. Application of force by muscle is always accompanied by shortening of muscle fibers according to the length-tension and force-velocity properties of muscle. Skeletal muscle has a unique length-tension property such that maximum tension is produced at lengths near the normal resting length and any shortening leads to a decrease in tension. A shortening of about 50% of the resting length will lead to zero tension. Such properties are matched by the mechanical properties of the ribs to lead to the normal absence of rib fractures during physiological chest compression. There is a type of artificial muscle known in the prior art as the McKibben muscle (Gaylord, U.S. Pat. No. 2,844,126) which has a remarkable similarity to this action of muscle including intercostal muscles. The McKibben muscle is pneumatically actuated by inflating a bladder. The special property is that inflating a bladder leads to shortening of the muscle unit. This is accomplished by placing the bladder within a expandable braided cylindrical mesh made with flexible but inextensible fibers set at an acute angle (about 28 degrees unexpanded) with respect to the long axis of the muscle unit. Fibers are braided in a biaxial braid sometimes referred to as “Chinese finger trap” braid. A commonly used fiber material is nylon. This angle increases to about 54 degrees (C. Chou and B. Hannaford IEEE Trans on Robotics and Automation 12: 90-102, 1996) at maximum expansion (maximum shortening) when the net muscle force along the muscle length is zero due to the constraint set by the inextensible fiber. This type of braided sleeving is used extensively in the electronics industry because of this ability of expanding or contracting around different sizes. The length-tension relationship of this artificial muscle has been found to be linear and very similar to natural muscle (Gordon et al. J. Biomechanics 39: 1832-1841, 2006). This leads to a decrease in total applied force as the actuator shortens and resultant decrease in mechanical stress on supporting structures. A nylon sleeved artificial muscle with an unexpanded length of 23 inches and fiber angle of 28 degrees will shorten to 15 inches (35% of relaxed length) when inflated to a maximum fiber angle of 54 degrees. A maximally shortened artificial muscle force decreases to zero just like natural muscle. A cylindrical shaped muscle would have a diameter of 0.75 inch unexpanded and about 1.25 inch for maximum expansion. Natural muscle force-velocity properties diminish force at high shortening velocities. This action is not similar to McKibben muscle, but can easily be mimicked by adding a mechanical damper in parallel to the actuator (C. Chou and B. Hannaford IEEE Trans Robotics and Automation 12: 90-102, 1996) or by the simpler procedure of using an orifice (pneumatic resistance) to control the rate of bladder inflation. These procedures are well known to those skilled in the art. Thus, the McKibben muscle can be and has been applied as an artificial muscle substitute with similar length-tension and force-velocity properties to natural muscle. There has been no prior use of the McKibben muscle to compress the thorax or abdomen or other body part. All prior applications of artificial muscle has been connected to limb motion or a non-medical mechanical shortening application. No prior use of the McKibben muscle has used the pressure generated by the bladder itself for any purpose other than shortening of the muscle unit.
Cyclical compression of the lower extremities (Arkans, U.S. Pat. No. 4,396,010) has long been used for preventing pooling of blood in patients with impaired circulatory condition (deep vein thrombosis). This involves the application of pressure to a cuff or bladder analogous to the Halpern et al. (Halpern, U.S. Pat. No. 4,928,674) device used for CPR except cuffs are inflated over a portion or completely around the body part. Typically, the foot, calf, and thigh or the arms are the body parts compressed. The intent of such devices is to simulate compression of the limb veins by muscle and take advantage of valves located in large veins to direct flow back to the heart. A major limitation of current devices used for long term repeated cuff compression is due to surface trauma which leads to surface ulcers in patients (Oakley et al. BMJ 316:454-455, 1998). In this case cuff compression was limited to the sole of the foot at a pressure level of 80-130 mm Hg for 1 second every 20 seconds. The repeated chafing due to uneven compression was the most likely cause of foot ulcers. While uneven compression might be avoided with circumferential cuff compression, the high pressure levels required can still be a cause of ulcers. Even recently developed devices (e.g. Barak et al., U.S. Pat. No. 6,494,852) involve cuff compression which differs from the far gentler compressive action of normal muscle contraction. Cuff compression applies a pressure and resultant force which is held constant no matter how much volume reduction is achieved. A relatively high pressure is typically selected in order to promote a high peak velocity of blood returning to the heart. The level of pressure is then much higher than what is needed for expelling most of the blood volume from the limb. For example, a pressure of 3 kPa (22.5 mm Hg) on the calf leads to about 80 ml of blood volume expelled with very little additional volume expelled for higher pressures (Thirsk et al. Med. And Biol. Eng. and Comput. 18: 650-656, 1980) So applying 80-130 mm Hg achieves a high peak velocity at the expense of surface stress which should be avoided. This limitation is proposed as a key factor in explaining poor tolerance by patients when cuff compression is applied for extended periods. Various methods of compression such as intermittent or sequential or amount of tissue compressed (calf versus thigh) have been proposed as improvements, but at present there is no evidence to support the superiority of one method over another (Proctor et al. J Vasc Surg 34: 459-64, 2001.) Sequential compression involves compression in a sequential order from the extremity of the limb toward the torso. Thus, the main problem with prior art remaining to be addressed is surface trauma.